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Southern California's upcoming rains could cause trouble for those living near hillsides, says Caltech geology professor Michael Lamb. That's because sediment gathered in riverbeds has been building up due to a lack of major storms.

Communities living near recent fires could face a similar problem, Lamb notes, since fires bake the soil dry. That turns the normally absorbent ground into a water-resistant surface, almost like concrete.

Large rainstorms send water down those hills with minimal absorption, meaning debris on the hill can get washed into communities below.

Eric Boldt with the National Weather Service says this storm is expected to drop a lot of rain in little time — the perfect conditions for this kind of problem.